1. Field of the Invention
The present invention relates to removing ammonia from a gas stream.
2. Description of the Prior Art
Fuel cells using acid electrolytes, such as phosphoric acid, suffer a performance decay when the fuel fed thereto contains more than 1.0 ppm, by volume, of ammonia gas. Fuels containing greater than 1.0 ppm ammonia may be cleaned to acceptable levels upstream of the fuel cells using a state-of-the-art non-regenerable bed of material which reacts with the ammonia. In a commercial fuel cell power plant designed to operate virtually maintenance free for about five years, it is certainly not desirable and can be quite expensive if the scrubber material must be replaced several times during this five year period.
In the prior art it is known that phosphoric acid reacts with ammonia to produce an ammoniated salt of the acid, and that a bed of phosphoric acid soaked porous support material can therefore be used to scrub ammonia from a gas stream. After a period of time there is insufficient unreacted acid to adequately scrub the ammonia. Prior to that time scrubbing is stopped and the bed is regenerated by increasing the vapor pressure of ammonia in the bed to greater than the vapor pressure of ammonia above the bed, which causes the salt to decompose back to the acid and ammonia. The ammonia is thereupon expelled or evaporated from the bed.
Increasing the vapor pressure of ammonia in the bed is usually accomplished by heating the bed to a temperature above the temperature used during the scrubbing process. For example, U.S. Pat. No. 3,859,417 teaches desorbing various gases from a bed of scrubbing material by raising the temperature "at least 20.degree. C., preferably 30.degree.-60.degree. C. above the maximum temperature at which absorption occurs" (Col. 9, 11 33-36). The scrubbed gas is then recovered. In Canadian Pat. No. 701,001 issued Dec. 29, 1964, scrubbed ammonia is recovered by raising "the temperature in the absorption by e.g. 100.degree. C." (page 5, 11 4-6). While always requiring at least an increased temperature for desorption, the Canadian patent also teaches that desorption may be accelerated by a carrier or sweep gas such as nitrogen, hydrogen, air or water vapor. The object, once again, is to further reduce the vapor pressure of ammonia above the bed relative to the vapor pressure in the bed.